Adjustable impedance transformer



Feb. 3, 1953 c. F. P. ROSE ADJUSTABLE IMPEDANCE TRANSFORMER 2 SHEETS-SHEET l Filed May l8, 1949 NVENTOR CER ROSE ATTORNEY Feb. 3, 1953 c. F. P. RosE 2,627,550

` ADJUSTABLE IMPEDANCE TRANSFORMER Filed May 18, 1949 4 2 SHEETS-SMT 2 3 Afm,- ll

I l n l a /i/VENTOR CEP. ROSE AT TORNEV Patented Feb. 3, 1953 Charles F. AP. Rose, Asbury Park, N. J., assigner to Bell Telephone Laboratories, Incorporated, :New

York, N. Y., a corporation offNew York t t Application May is, 1949, seriamo. 94,013

` l 5 claims. (ci. 17e- 44) The invention relates to high frequency transmission systems, and particularly to impedance transformers for use in such systems totransform the' impedance of one' transmission device or line therein so that it may be electrically coupled'to another transmission device or'line having an impedance of dierent valueor typewithout the "introduction of Vdetrimental transmission effects.

The invention is applicable, for example, to a transmission line coupling between anoscillation generator or other source of high frequency elec-v tric waves, such as a magnetron, having an impedance embodying both reactance and resistance components, and a radiating antennaor other load device having a highly resistive impedance. It is known thateicient transmission of high frequency power from one such device to another requires` that the devices be properly matched in impedance so as to avoid the production of standing waves with consequent increase in transmission losses, due to reflections at junction points or other causes. For this purpose, it is usual to insert some sort of transformer in the connecting line to transform the impedancecou pled to one end of the line to a different value of impedance at the other end. Such transformers may comprise transmission lines of either ixedor adjustable lengths. In certain known systems, they have been in the form of an additional -section or sections oi transmission line having an effective length equal to a quarter of a wavelength (i/4)',or an odd multiple` thereof,'of the wave tobe transmitted, and of characteristic impedance suchras to provide the desired transformation ratio. Y

A general object of the invention is to improve transformers of the above-described general type reduction-inmanufacturing costs. l 4 Another object is to efficiently couple transmission devioes of dierent impedance characteristics without introducing appreciable transmis sion losses. l t

*A more specific object is to match the impedance of a source of high "frequency oscillations, such as a magnetron, having an-internal impedance embodying resistance different from that of the usual line, and also reactanceQ to that of' a load, `such -as a radiating antenna, whichis ',1

highly resistive.

`The impedance transformers in accordance withfthe'invention are of the adjustable coaxial or wave guide line -type., jr1nl one embodiment of the invention, the impedance transformerconf fromthe standpoint of simplicity, flexibility andl sists of a tapered (conical)` metalcore member. 2%; Wavelength longslidable longitudinally along" the inner conductor of a coaxial line connecting ra magnetron or other reactive'high frequency4 l wave generator tojan antenna' or other resistive load device, and an outer metal' shell member in wavelengths long' slidable longitudinally 'along the inside of the outer conductofof the connectr ing coaxial line. The inside 'of' the outer shell member is correspondingly tapered at one end for 3A, wavelength so lthat in one adjusted posi tionit closely surrounds the core member, and is cylindrical for the remainder 4of its length, Means are provided for manually sliding both members simultaneously'with'a `iix'ed longitudinal relationship between them, longitudinally along the coaxial line towards orl away from the reacn tive generator until an adjustment point is found at which the reactance component of the internal impedance ofthe generator is effectively neutralized; and for independently sliding one of thev two members longitudinally valong the line in either direction. to so adjust its longitudinal;

proximity with respecty tothe other memberas to match or equalize` the vresistance"components of the impedance of the generator and antenna or other load device. u i

In another embodimentof the invention, the impedance transformer` comprises a hollow metal housing of circular inner cross-section connect ingv two sections of air-dielectric coaxial lline reu spectively, extending to the reactive high fre-4 quency generator and to the antenna or other resistive loaddevice; two cylindrical tubular or sleeve members of conductive material, each 1/4 wavelength longand having innert and-outer', diameters properly dimensioned with respect to the outer diameter of the inner coaxial line-y conf ductor and the insidediameter of the/outercof axial line conductor, respectively, to provide the` required characteristic 4impedance Lfor the trans-g former, which merrlbersare mounted in cascade relation` with each other coaxially within the housing; and suitable. gearing and associated locking meansI iol" causing the twocylindricaltubular members, each of which in itself is a members from the, "generator to an optin'uirn point at which the 're'actance'oftheY internal irnpedance of the latter is effectively neutralized; and the independent adjustment of one or the other of the two members may be used to adjust the longitudinal proximity of that element with respect to the other so as to effect the required impedance transformation between the send and receive ends of the cascaded members to match or equalize the resistance components of the impedance of the generator and antenna or other load device.

Figs. `2 and 3 show, respectively, a plan and a front elevation assembly view of another embodimentv of the impedance transformer of the invention; and

Figs. 4 to 7, inclusive, show respectively sectional views taken along the lines. 4 4, 5 5, and 7-1 of the assembly of Fig. 2 to illustrate various structural details of the second 'embodi-l ment of the invention.

Fig. l shows one embodiment oi the impedance transformer of the invention applied to a coaxial line having inner and outer concentric conductors l and 2, respectively, connecting a high frequency generator M of the magnetron type to a radiating antenna A in a radar system. The generator M may be, for example, a magnetron vsuch asy is disclosed inthe United States patent of A. L. Samuel, No. 2,063,343, issued December 8, 1938, and the antenna A may be of the coaxial lineA type, such as disclosed, for example, in the United StatesV patent of W. H. C. Higgins and C. A. Warren No. 2,424,982, issued August 5, 1947. Such a magnetron mayr have an internal irn- Dedance somewhat different from that of the coaxial line, and alsol reactance; whereas the antenna may have an impedance which isI predomi- I nantly resistive.

The impedance transformer of Fig. l includes a.

central. core` member 3 and an outer shell member 4 both made from a conductive material, such as brass. The core member 3, which is of frustoconical shape and has a length which is 3A; of a wavelength (3A/4l of the wave to be transmitted from magnetron M to antenna A, is mounted with its thicker end nearest the lower impedance, represented in this case by the magnetron M, coaxially within the outer cond-uctor 2 of said coaxial line, by means of a central opening of constant diameter along its longitudinal axis (axis of member 3) on the inner coaxial line conductor I so as to be slidable longitudinally therea-long in either direction. The shell member 4, which is cylindrically shaped on the outside and has a length which is f3/4 of a wavelength iA/4) of the Wave tobe transmitted, is arranged to beV slidable longitudinally in either direction along the inner surface of the outer coaxial line conductor 2. The inside of the shell member 4- is of circular cross-section and is tapered at one end nearest the magnetron M for 3A of a wavelength ('37\/'4) so that for one adjusted position ofthe two members 3 and 4 with respect to each other along the coaxial line, the tapered portion of member will closely surround the irustoconical 'core member 3 throughout the entire 4 length (3A/4) of the latter, and is cylindrical for the remainder of its length.

As shown, the outer diameter of the frustoconical core member 3 at its thicker end equals the inner diameter of the cylindrical portion of the outer shell member t, and the outer diameter of the member 3 at its other end is slightly larger than the outer diameter of the inner coaxial line conductor E. The core member 3 is recessed slightly on the inside for a given length at its thicker end, and this recessed portion is provided with one or more circumferential rows of phosphor bronze spring iingers 5 with coin silver ends to hold the member 3 in goed contact with the` cuter surface of the inner coaxial line conductor l. The cuter surface of the outer shell 4 at its thicker end, is similarly recessed foil a substantial portion or" its length, and one or more circumferential rows of phosphor bronze spring fingers 6 with coin silver ends are attached thereto to provide good electrical contact of the outer shell member i with the inner surface ol' the outer coaxial line conductor 2 at all times. Preferably, as shown, the longitudinal recess in the outer surface of shell member 4 extends to the center thereof to provide. adequate contact at the current maximum` point and thereby reduce sparking. The other ends of the two slidable members 3 and 4 contain longitudinal. slits (not shown) to provide enough flexibility of these members to maintain the associated springl ngers in close contact, with the outer surface of the inner coaxial line conductor I and the inner surface of the outer coaxial line conductor 2, re-

f spectively, when. the two members 3 and 4 are moved longitudinally in either direction. A long slot l' is out in the side of the outer coaxial linc conductor 2, at an intermediate point of the coaxial line, and transverse apertures e andi] are provided, as. shown in the tapered portionl of the outer shell member Il near its thinner end, and in the inner core member 3 near its thicker end, respectively.

The manner of adjustment f the impedance transformer of. Fig. l so that it will match the impedances of the magnetron` M and the antenna A to a high degree of accuracy will now be described.

With the members 3 and 4 initially positioned so that the apertures 9 and 8, respectively, there in are aligned vertically with each other and an intermediate point along the slot 1 in the outer coaxial line conductor 2,y theoperator will insert the end of a rod I0 of insulating materiaL Such as polystryrene, through slot 1 and both apertures 8 and 9. The operator will then push the rod l0 to the left or right against the sides of apertures 8 and 9 so as to slide both members 3 and 4 as a unit longitudinally along the associated inner and outer coaxial line conductors and 2, respectively, towards or away from. the magnetron M until these members are located at the optimum distance from the magnetron` at which the reactance component of the latter is eiiectively neutralized. When this point is found, the inner core member 3 is maintained stationary in its adjusted position, and the end of the rod I is withdrawn from aperture 9 in the core member 3 but is maintained within the aperture 8 in the outer shell member 4. The operator will then push the rod to the. left or right along the slot 'l so that it will bear against the side of aperture 8 in shell. member 4 causing that member to slide longitudinally along the inner surface of the outer coaxial line conductor 2 nn.-

encaisse.,

til its longitudinalproximity with respect to the stationary core member 3 is properly adjusted to match or equalize the resistance components of magnetron M and the antenna, A.

The optimum adjustment of the position thecore member 3 and shell member 4 of the impedance transformer of Fig. 1 may be determined by trial and error measurements of the standing wave ratio (ratio of maximum to minimum voltage) within the coaxial line for different adjustments of the members. When the observed standing wave ratio is a minimum the impedances of magnetron M and its antenna A are properly matched. Such measurements may be made with a standing wave detector of any of the well-known types, for example, one employing a- -movable probe within the coaxial line,

coupled through a coaxial jack in the side oithe line to a measuring circuit employing a crystal detector,` an amplifier and an oscilloscope in tandem.

The characteristic impedance of any portion of the impedance transformer shown in Fig. 1 may be determined from the following conventional formula applicable to an air-dielectric co-` axial line: I

d=the outer diameter of the inner conductor of the coaxial line.

The characteristic impedance of the coaxial line is The impedance Z1 at'the point of the largest diameter of the core member 3 when the members -3 and 4 are positioned as shown in Fig. 1 is i .977 The transformation ratio N of the transformer equals Z0=1 lOgm 'iPass-2 In the test referredto abo-ve, a position was found for adjustment o-f the inner core member 3 and outer shell member 4 of the experimental transformer which presented an optimum im- (antenna) beyond the. transformer. showed also that movement of the .inner core member `3 a 1/2 wavelength (M2) `closer to thc magnetron, gave essentially the same` results, i. e.,

the same impedance Z was presented to the mag- Y netron and the same powerwas delivered to the load. The latter adjustment position was advantageous in that it permitted operation of the impedance transformer of Fig'. 1 at positions which will not; have the undesirable-breakdown voltages which it has when the two members are overlapped in position.

The other embodiment of the impedance transformer of the'invention illustrated in Figs.

2 to 7, inclusive, includes a hollow housing II,. which is shown, as being of square cross-section but may be of circular, rectangular or other suit-- able cross-section, connecting the two sections LI.

and L2 of coaxial line having inner and outerconcentric` conductors I and 2, respectively, lead-Y ing t0 the magentron MA and the antenna A, re

spectively.` The inside IIa of housing Il, which. is cylindrically shaped and may be of the` same..

circularcross-section as the outer conductor 2,

of the coaxial line forms a connecting link between the portions ofthe outer coaxial line con-- pedance tothe, magnetron and, therefore, al-

lowed maximumpower to be. supplied to the load-- ductor 24 in line sections LI and L2 extending throughthe end plates I2 and I3, respectively. of

that housing as indicated in Fig. 3; Two cy1in drical tubular or sleeve members I4 and I5 of a` conductive material, such as brass, each havinga` length of 1/4 of a wavelength (M4) of the operat ing frequency of the magnetron M and inner and.

outer diameters determined by the desired char-- acteristic impedance of the transformer, are mounted coaxially within the housing VII in cas-r cade relation with each other in the manner to be described. Y

Two metal rods I6 and I1 extend in parallel with each other and the longitudinal axis of the housing II between and are iix'edly attached to the block I8` attachedto and projecting vertically above Lthe `top of housing II at one end thereof an'd a fixed bracket ISofinverted-U shape comprising an upper, transverse arm I9a and two ver-A tical legs ISb ,and Iv9c `extending downwardly therefrom on Oppositesides of the housing, which is attached to and extends above the top of hous-` ing II at its other end. Two,y movable brackets 2D and 2I also of inverted-U shapehave their upper transverse arms 20a.` and 2Ia slidably mounted on the rods I6 and I1 and their two vertical legs20b and 20c and 2lb and 2Ic, respectively, of suitable length extending downwardly from the transverse arms on opposite sides of the. outside of housing Il. `Four members 22, 23, 26 and 21 of dielectric material, such as polystyrene, are employed for supporting the tubular members I4 and I5 in proper position within the housing `I I. Two of the dielectric members 22 'and' 23, which have their outer ends respectively movement of the `movable bracket 20 along the rods1l6 and I1. Similarly, the two dielectric members 2li and 27, whiclrhave their outer ends The test respectively aiiixed to the bottom. of a di'erent one ofthe two vertical legs 2Ib: and 2Ic of the movable bracket 2 I, extend transversely through the longitudinal slots 24 and 25, respectively, in opposite sides of the housing II into the interior thereof, where their other ends` are respectively attached at diametrically opposite points to the circumference of the tubular member I5 so as to support that member in coaxial relation With the housing II when it is displaced longitudinally therein in either direction by slidable movement of the movable bracket 2-I along the' rods I6 and II.

The longitudinal movement of the movable brackets 2t and 2I along the rods I6 and II is controlled by a gearing arrangement comprising a rack Ziixedly attached at` one end to the transverse arm a of the movable bracket 20 and extending, inparallel with the longitudinal axis Vof housing II, throughr suitable openings inV theA transverse. arms 2Ia and IS'a ofthe movable bracket ZI and the xedendbracket I3, respectively, and two pinion gears 29 and 30 meshing with the rack 28. The shafts of the gears 29 and 30 are respectively journaled for rotation between the vertical legs 2Ib'and 2'I-c of the movf able bracket 2I and the vertical legs I9b and I9c of the fixed bracketl I9 under respective` control of the knurled knobs 3lv and 32 respectively af"- xed to'one end of the shaft of gear 29 and to one end ofthe shaftof gear 30, to cause longitudinal. movement of the rack 28 in one direction or they other.

A metal strap 33 having a longitudinal slot 34 therein, has one end allixed to the vtop of the transverse arm 20a` of movable bracket 2i) and extends acrossY the tops of the'transvers'e arms Bia and Ita of movable bracket 2I and the fixed end bracket I9, respectively. A set screw-35 having a Wing nut top, inserted through the slot 34 in strap 33 is adapted to be screwed into a threaded hole in thetop of the transverse arm 2'Ia of movable bracket 2I to clampthe latter tostrap 33 at a desiredpoint alongr slot 34'; and a similar set screw 36V havingl a wing nut top, inserted through the slot 34-'in strap 33 is adaptedV tobe screwed into a threaded hole in thetop of the transverse arm I9a of the xed bracket I9 to clamp the latter to strap33 at a desired point along slot 34.

The manner of adjustment ofthe impedance transformer of Figs. 2 to '7, so that it will match the impedances of the magnetron M andv the antenna A respectively connected to theiopposite ends of the transformer through coaxial line sections LI and L2 to a high degree of" accuracy, will now be described.

With the set screw 35- loosened so as` to un-r clamp` the xed bracket I9 from strap 33 and:

thev set screw 35 tightened t'o clamp movable bracket 2l to strap 33, the operator will rotate knob 32 attached to the shaft of pinion 30fto the left or right to cause longitudinal movement of the rack 28y and thus of the' movable brackets 20 and 2l, now effectively aixed thereto, to the'left or right, so as to move both tubular members` I4v and I5 supported by movable brackets 20v and 2|', respectively, together with a xed longitudinal distance between them, longitudinally towards or away from the magnetron M- until the members I4 and I5 are locatedat the optimum distancel vfrom the magnetron at which thefreactance'component of the latter is effectively neutralized; When; this point is found, the set screw 36 is .tightened to clamp- Xedj bracket I3 tothe` strap 33, so as tov prevent further movement of the rack 2B' and the movable bracket 20' and thus of tubular member I4 attached thereto and maintain the adjusted longitudinal position of the members I6' and I5'with respect to the magnetron M at which the reactance of the latter is neutralized. The operator will then loosen set screw 35 to unclamp movable bracket 2I from strap 33 and will rotate the knob 3I1 attached to the shaft ofpinion gear 2.9 to' the right or left to cause a corresponding right or left rotation of gear` 29. This will move the movable bracket 2|, because the latter is now' unclamped from strap 33 with set screw 35 loosened, and thus the tubular member` I5 within housing II, to the right or left until the longitudinal proximity of' the4 tubular' member I5 with` respect to the stationaryv tubular member I4 is properly ad justedv to match or equalize the resistance components of the magnetron M and antenna A. The set screw 35 will then be tightened so as to clamp the movable bracket 2i. to the strap 33 and maintain the relative' longitudinal position of the tubular members I4 and i5'within the housing II at the optimum point found. Y

The optimum adjustment of the positions of the tubular members Ifl and I5 of the impedance transformer of Figs. 2 to 7, as in the case of the impedance transformer of Fig. l. may be determined by a series of trial and error measurements of the standing wave ratios within the housing II for different adjustments of the members, using a standing wave detector.

The mean diameter of each tubular member I4 and I5 is made equal to the geometrical mean of the diameters of the inner and outer concentric conductors I and 2 of the coaxial line. This was done to accommodate the greatest permissible irregularity in alignment of the two members I4 and I5 in assembly of the transformer. The other dimensions of the membersjvlll and I5 were made such as to satisfy the following well-known equations ZE: 138 10g10g+10g (3) where Ziathe characteristic member I4 or l5; a=outer diameter of inner line conductor I; b=inner diameter of tubular member I4 or I5; c=outer diameter of tubular member I4 or I5; d=inner diameter of outer line conductor 2.

impedance of tubular receiver end of the transformer, an impedance ZD 1s seen, where Zfi ZAS i ZL- ZC Ol -Z-B In the practical embodiment of the impedance transformer of the invention shown in Figs. 2 tou?, whichY was constructed for use with` acoaxialline` having an inner lconcentricv conductor of 0.625 inch diameter and an outer concentric conductor of 1.527 inches diameter, the inside diameter and outside diameter of each tubular member where 0.914 and 1.039 inches, respectively, and the design of the elements determining the range of longitudinal adjustment of the members lli and I was such as to allow a minimum longitudinal spacing between them of not less than a 1/4 wavelength and a maximum longitudinal spacing between them at least a 1/2 wavelength beyond that point.

A unique feature of the impedance transformer of Figs. 2 to 'I is that the mounting of the tubu lar members i4 and I5 Within the housing ii is such that they do not make metallic contact with the inner wall of the housing during adjustw ment of the position oi these members thereby eliminating the sparking difficulties involved in transformers utilizing sliding of the variable elements along metal surfaces.

Various modifications of the impedance transformer arrangements illustrated and described which are within the spirit and scope or the invention will occur to persons skilled in the art.

What is claimed is:

1. An adjustable impedance transformer comprising a section of coaxial line having inner and outer concentric conductors, a metal shell member having a length equal to an odd number of 1A wavelengths of the design frequency, adapted for adjustable sliding contact longitudinally along the inner surface of said outer concentric line conductor. the inner surface at one end of said shell member being tapered over a portion of its length, a conically shaped metal core member having a corresponding taper, a longitudinally extending hole through said core member for accommodating the inner conductor of said section of coaxial line, said core member being adapted for adjustable sliding contact longitudinally along the outer surface of said inner concentric line conductor, means for establishing said members in fixed longitudinal relationship with respect to each other and for moving said mem bers in said xed relationship in either direction along said coaxial line to a desired position and means for producing independent adjustment of the longitudinal proximity of one of said members with respect to the other to vary the im pedance transformation ratio of said transformer.

2. A transformer for coupling two transmission devices having different impedances at least one of which includes both reactance and resistance components, comprising a section of coaxial line having inner and outer concentric conductors, a metal core member of frusto-conical shape having a length which is 3A. of a wavelength at the frequency of the wave to be transmitted between said devices, a hole extending longitudinally through said core member for accommodating the inner conductor of said section of coaxial line, said core member being coaxial with and slidable longitudinally on said inner concentric conductor, a metal shell member 54 of a wavelength long at said frequency, coaxial with and slidable longitudinally along the inside of said outer concentric conductor, said shell member being conically shaped inside at one end for wavelength so that for certain adjusted positions of the two members it closely surrounds said core member, and being cylindrically shaped nside for the remainder of its length, means for establishing said members in xed longitudinal relationship with respect to each other and for sliding both members in said xed longitudinal arrangement to a position along said line at which the reactance components of the im pedances of said devices are eiectively neutralized and means for separately producing sliding adjustment of the longitudinal proximity of one of said members with respect to the other so as to effectively match the resistance components of the impedances of said devices.

3. The transformer of claim 2, in which the largest diameter of the frusto-conically shaped core member equals the inside diameter of the cylindrically shaped portion of said outer shell member and is proportioned with respect to the outer diameter of said inner concentric conductor and the inner diameter of said outer concentric conductor to provide the desired characteristic impedance for the transformer.

4. The transformer of claim 2, in which one or more circumferential rows of spring iingers with coin silver ends are attached to recessed portions in the surfaces of said core member and said shell member at the thicker ends thereof, nearest said inner and outer concentric conductors, respectively, and longitudinal slits are provided in the other ends of said members to give the required amount of flexibility thereto, in order to provide good contact between the slid ing surfaces.

5. The transformer of claim 2, in which the last-mentioned means comprises a long slot in said outer concentric conductor, an aperture through said shel1 member near its thinner end, a second aperture in said core member near its thicker end and a rod adapted for insertion through said slot and both of said apertures to slide both of said members longitudinally along said coaxial line, or alternatively through said slot and the aperture in said shell member only to slide the latter member longitudinally with respect to said core member.

CHARLES F. P. ROSE.

REFERENCES CTED The following references are of record in the le of this ipatent:

UNITED STATES PATENTS Number Name Date 2,403,252 Wheeler July 2, 1946 2,408,745 Espley Oct. 8, 1946 2,435,442 Gurewitsch Feb. 3, 1948 2,473,262 Scheldorf June 14, 1949 2,543,721 Collard et al Feb. 27, 1951 OTHER REFERENCES Microwave Transmission Circuits edited by Raglan, volume 9, iirst edition of the Radiation Laboratory Series, copyright May 21, 1948, published by McGraW-Hill Book Co., Inc. (Copy in Patent Ofi-ice Library.) 

